Conventional electrical machines, such as generators and motors, use a magnetic stator core to provide a low reluctance path for the magnetic fields created by the excitation source. The outer diameter of the stator core is formed of a continuous ring of electrical steel (known as the core-back) of sufficient radial depth to ensure, by design, the magnetic permeability is high. As a direct result, very little magnetic field exists outside the outer diameter of the stator. In this respect the core-back acts as an effective environmental magnetic screen for the magnetic fields generated and contained within the machine.
However, superconducting electrical machines operate at significantly higher magnetic fields levels than non-superconducting electrical machines. The high field levels mean that a conventional stator core-back saturates magnetically causing its magnetic permeability to reduce so that significant magnetic fields exist outside the stator core. This means that existing core-back designs are not effective for such superconducting electrical machines, leading to the presence of significant magnetic field levels external to the outer diameter of the machine. This represents an environmental hazard in the vicinity of superconducting machines.
It is also possible that non-magnetic stator cores may be employed in superconducting electrical machines, in which case an environmental screen is almost certainly a necessary requirement.
Conventional design techniques may be used for superconducting electrical machines, for example by increasing the size of the core-back. But this leads to a significant increase in the radial depth of the required core-back, resulting in an undesirable increase in the size and weight of the machine.
Passive screening could be used either in the form of a high permeability ring of material such as steel or a high electrical conductivity material such as copper or aluminium.
However, such passive screening adds significant weight and/or electrical losses to the superconducting machine, which are undesirable.